An Extreme defence flask is a potion flask originally containing six doses of Extreme defence. It is created by using extreme defence on a potion flask. Drinking a dose will temporarily raise the player's Defence level by 3 + 15% of the player's Defence level (rounded down).

To decant the potion into the flask, one must have at least 6 doses of extreme defence in their inventory, as well as one or more potion flasks. Using the flask on the potion will open a menu to decant them. The optimal inventory setup for decanting potions is 17 4-dose potions and 11 flasks.

Once the potion has been partially drunk, no more doses can be added to it. After all six doses have been consumed, the flask will shatter and disappear. Flasks can be combined together, but the empty flasks will still be destroyed afterwards.